In many types of structural members it is a requirement to know what force can be applied to the member before the member will fail or "yield". Such "yield" point can be easily calculated when the members are under static load conditions; however, when a member is constantly subject to a fluctuating movement where the fluctuations establish some sort of a definite pattern a different kind of failure is applicable, such being defined as a fatigue failure. A fatigue failure is usually a gradual or progressive fracture. The fracture starts at some point in the structural member at which the stress is much larger than anticipated. This high localized stress causes a small crack which gradually separates as the flexing movement is repeated until the entire member ruptures without measurable yielding. It is well known in this type of failure that the inclusion of an aperture therein, such as a drilled hole for the insertion of a fastening member, creates an area for the derivation of a high localized stress. It is desirable to modify the aperture in some manner whereby the high localized stress can be either substantially reduced or eliminated.
In aircraft, during flight, the wing structures and the fuselage structural members are subjected constantly to this type of fatigue failure. The wind turbulence, coupled with the aircraft maneuvers, cause a cyclic movement of practically all the structural supporting members within the aircraft. As such members usually contain apertures therein for the insertion of fastening means, the high localized stress created thereby greatly compounds the fatigue failure problem.
A method to increase the fatigue and endurance limit of structural members is described in U.S. Pat. No. 3,110,086. The method disclosed comprises the forming of a thin groove spaced from and around an aperture in the structural member. Although this method has been of great advantage for the past number of years, such still does not solve the problem of decreasing the high localized stress, but merely creates a compressive stress which functions as a fence around the area of the localized stress.
Another method to increase the fatigue and endurance limit of structural members is described in U.S. Pat. No. 3,434,327, filed Nov. 1, 1966 by the Applicant, for Stress Coining, which was copending with the original patent application of this continuation. The application of this method results in metallic material in the vicinity of the aperture, or area of high localized stress, to be compressed to such a point as to cause the material to exceed the elastic limit and become plastic. The force required to accomplish such must be substantial to cause sufficient material displacement and deformation within the plastic range of the material. Upon release of the force the material will rebound back into the elastic range with the result that a residual, compressive, congenital stress is obtained in the area of the aperture which tends to counteract any stress which might cause failure of the structural member. The effect of the applied force also causes the elimination of any surface cracks or other breaks in the material due to the drilling or other forming operation of the aperture. Also, any undesirable tensile stresses are substantially reduced or eliminated. This method is known as the Stress Coining Method.
The material displacement to effect the operation of the method disclosed in the above-identified copending application must be within a certain range in order to accomplish the desired results. The material displacement, if not sufficient, would not cause the elimination of the surface stresses and also would not create sufficient residual compressive stress. If the material displacement is too great the rebound of the material will be slight in proportion to the material displacement; therefore, not creating the maximum value of residual compressive stress in a wall of the aperture. It has been found through experimentation that a rebound of approximately one-half the value of the initial material displacement is satisfactory. The stress coining tool of the present invention accomplishes the above objectives with great assurance.
There have been other attempts to solve the problem of fatigue failure. It is known that the mere inclusion of a screw slightly increases the fatigue endurance limit of a structural member having an aperture therein. The inclusion of a lockbolt or rivet slightly improves the fatigue characteristic over the application of a screw. However, still greater advances could be achieved in the area of fatique failure.
Conventional screws and bolts include various types of configurations for accomplishing various results with respect to an adjacent wall portion such as sealing, sizing, broaching, burnishing, etc.; however, no prior fasteners have the capability of stress coining or coldworking the wall material of the aperture to accurately size the aperture, to eliminate surface cracks and undesirable tensile stress, and also to create residual compressive stress to counteract fatigue tensile stresses which tend to cause premature failure of the structural member.
Since the inclusion of an aperture in a structural member is usually for the insertion of a fastening member, it is a direct improvement on the state-of-the-art to provide a fastener with the features of the aforementioned stress coining tool which is capable of stress coining the aperture when the fastener is installed. Such a fastener eliminates the additional work of stress coining the aperture itself as a separate operation. Thus, in one operation the desired modification to the aperture can be made whereby the high localized stress can be either substantially reduced or eliminated. The fasteners of the present invention accomplish the above objectives with great assurance.